1. Field of the Invention
The present invention relates to an electronic device having a CMOS circuit, and more particularly to an electronic device having a CMOS circuit for which a low voltage operation is required.
2. Description of the Related Art
FIG. 4 is a schematic circuit diagram showing conventional voltage detecting circuit. As shown in FIG. 4, the conventional voltage detecting circuit is mainly made up of p-channel MOS transistors (hereinafter referred to as xe2x80x9cPMOSxe2x80x9d), n-channel MOS transistors (hereinafter referred to as xe2x80x9cNMOSxe2x80x9d), a depletion type n-channel MOS transistor (hereinafter referred to as xe2x80x9cD-type NMOSxe2x80x9d) which allows a drain current to flow even when a potential difference between the source and the gate is 0 V, and resistors. A first PMOS 304 has a source connected to a power supply terminal 101 and a drain connected to its gate, a drain of a D-type NMOS 305 and a gate of a second PMOS 306, respectively, and the D-type NMOS 305 has a source connected to its gate and a GND terminal 102. Also, the second PMOS 306 has a source connected to the power supply terminal 101 and a drain connected to a drain of a first NMOS 307 and a gate of a second NMOS 311, and the first NMOS 307 has a source connected to the GND terminal 102 and a gate connected to a second electrode of a first resistor 109 and to a first electrode of a second resistor 108, respectively. A first electrode of the first resistor 109 is connected to the power supply terminal 101, and a second electrode of the second resistor 108 is connected to the GND terminal 102. Also, an output resistor 110 has a first electrode connected to the power supply terminal 101 and a second electrode connected to an output terminal 103 and to a drain of the second NMOS 311, and a source of the second NMOS 311 is connected to the GND terminal 102.
Also, the power supply terminal 101 is connected to a plus side terminal of a power supply means such as a battery, and the GND terminal 102 is connected to a minus side terminal of the power supply means.
In the conventional voltage detecting circuit thus structured, a voltage detecting portion is made up of the first PMOS 304, the second PMOS 306, the D-type NMOS 305 and the first NMOS 307, a bleeder resistor portion is made up of the first resistor 109 and the second resistor 108, and an output portion is made up of the output resistor 110, the output terminal 103 and the second NMOS 311. Also the bleeder resister portion outputs a divided voltage generated by dividing the voltage of the power supply terminal 101 by the first resistor 109 and the second resistor 108, and the voltage detecting portion detects the voltage of the divided voltage, to thereby indirectly detect the voltage of the power supply terminal 101. The output portion reflects the detection result of the voltage detecting portion on the voltage of the output terminal 103.
In addition, FIG. 2 shows a graph of the voltage of the output terminal 103 (hereinafter referred to as xe2x80x9coutput voltagexe2x80x9d) to the voltage of the power supply terminal 101 (hereinafter referred to as xe2x80x9cpower supply voltagexe2x80x9d) of the voltage detecting circuit. A bold line and dotted line portion shown in FIG. 2 is a graph of the above-mentioned conventional voltage detecting circuit. A bold line and thin line portion is a graph of a voltage detecting circuit according to the present invention which will be described later. As is apparent from FIG. 2, it is found that when the power supply voltage drops from a state where the output voltage is the power supply voltage, the output voltage becomes the voltage of the GND terminal (hereinafter referred to as xe2x80x9cGND voltagexe2x80x9d) with a boundary of a given power supply voltage. However, in the case where the output voltage originally drops down to the GND voltage, even if the power supply voltage further drops, the output voltage must naturally maintain the GND voltage. However, when the power supply voltage reaches a given voltage or lower, it is found that there occurs such as phenomenon that the output voltage becomes higher than the GND voltage. A region of the power supply voltage where such a phenomenon occurs is called xe2x80x9cindefinite regionxe2x80x9d, which is caused by a fact that the conventional voltage detecting circuit structured as described above cannot operate due to a drop of the power supply voltage. Also, the above-mentioned conventional voltage detecting circuit enters the indefinite region from a high power supply voltage of about 0.6 V.
The above-mentioned conventional voltage detecting circuit suffers from a problem in that, in an electronic device having the above-mentioned conventional voltage detecting circuit and a load circuit which is driven by a power of the above-mentioned power supply means, the load circuit is reset by using the output voltage of the above-mentioned conventional voltage detecting circuit.
In this case, since the power supply voltage of the load circuit drops with a drop of the voltage of the power supply means such as the battery, when the power supply voltage of the load circuit becomes a given voltage or lower, the load circuit conducts unstable, resulting in a problem in that the electronic device is fatally damaged in a system or hardware fashion.
In order to prevent this problem, it is necessary that in the case where the power of the power supply means becomes a voltage or lower at which the load circuit conducts the unstable operation, the conventional voltage detecting circuit drops the output voltage to the GND voltage from the power supply voltage so as to reset the load circuit, and the conventional voltage detecting circuit maintains the output voltage to the GND voltage until the power of the power supply means becomes a voltage or lower at which the load circuit cannot operate at all, to thereby maintain the reset of the load circuit.
However, the conventional voltage detecting circuit is high in the power supply voltage that enters the indefinite region. Therefore, in the conventional voltage detecting circuit, the power of the power supply means is caused to enter the indefinite region at a voltage which is higher than the voltage at which the load circuit cannot operate at all. For that reson, since the reset of the load circuit is canceled by the power supply voltage at which the load circuit conducts the unstable operation, the problem in that the load circuit is fatally damaged in the systematic or hardware fashion cannot be prevented. Also, this problem becomes increasingly severe since the power supply voltage region where the load circuit unstably operates is also lowered in voltage with the lower voltage operation of the load circuit in recent years.
In order to prevent the above-mentioned problem, in the conventional voltage detecting circuit, there may be applied a method in which the threshold voltages of the respective PMOSs and the respective NMOSs shown in FIG. 4 are lowered so as to drop the power supply voltage that enters the indefinite region. However, in this case, the leak currents of the respective PMOSs and the respective NMOSs increase with the result that there occurs a problem in that the current consumption of the conventional voltage detecting circuit applying the above method increases.
That is, summarizing the above contents, the conventional voltage detecting circuit has an object that the power supply voltage that enters the unstable region is dropped while preventing an increase in the current consumption in order to prevent the above-mentioned problems.
The above description is given of the problems on the voltage detecting circuit; however, it is needless to say that a CMOS circuit having other functions suffers from common problems in the case where the low voltage operation is required.
The present invention has been made under the above-mentioned circumstances, and therefore an object of the present invention is to provide a voltage detecting circuit which is capable of eliminating the problems with the conventional voltage detecting circuit.
To achieve the above object, according to a first aspect of the present invention, there is provided an electronic device including a CMOS circuit having a desired function, and a power supply means for supplying a power that drives the CMOS circuit, in which the CMOS circuit has at least one internal circuit structured by a MOS transistor of a complete depletion type SOI structure, in which a depletion type n-channel MOS transistor is disposed between the plus side terminal of the power supply means and the plus side power supply terminal of the one internal circuit, which suppresses a source voltage to a desired voltage or lower in the case where a drain voltage is a predetermined voltage or higher, and which gives a gate voltage so that the source voltage becomes equal to the drain voltage in the case where the drain voltage is lower than the predetermined voltage, and in which the one internal circuit is driven by the power of the power supply means which is supplied from the drain of the depletion type n-channel MOS transistor to the source thereof. Accordingly, there can be adopted the power supply means that supplies a power having a high voltage, and also it is possible to realize an electronic device having the CMOS circuit which can operate even if the voltage of the power of the power supply means is lowered and which is low in the power consumption.
Also, according to a second aspect of the present invention, there is provided an electronic device in which the CMOS circuit is a voltage detecting circuit which has a function of detecting a voltage of the plus side terminal of the power supply means, in which the voltage detecting circuit includes an output terminal that outputs a voltage detection result, a voltage dividing resistor portion that outputs a divided voltage resulting from dividing the voltage of the plus side terminal of the power supply means, a voltage detecting portion that detects the divided voltage and reflects the detection result on an output signal, and an n-channel MOS transistor of a complete depletion type SOI structure a gate of which receives the output signal, and has an output portion that fluctuates the voltage of the output terminal, in which the voltage detecting portion is structured by a MOS transistor of a complete depletion type SOI structure, and the depletion type n-channel MOS transistor is disposed between the plus side terminal of the power supply means and the pulse side power supply terminal of the voltage detecting portion, in which the voltage detecting circuit is driven by using the power of the power supply means which is supplied from the drain of the depletion type n-channel MOS transistor to the source thereof, in which the depletion type n-channel MOS transistor is also disposed between the drain of the n-channel MOS transistor of the complete depletion type SOI structure of the output portion and the output terminal, and in which a drain current of the n-channel MOS transistor of the completion depletion type SOI structure flows between the drain of the depletion type n-channel MOS transistor and the source thereof. With this structure, there can be adopted the power supply means which supplies a power having a high voltage, and also it is possible to realize the electronic device having the voltage detecting circuit which not only reflects the voltage detection result on the output terminal even if the voltage of the power of the power supply means is lowered, but also is low in the power consumption.
Further, according to a third aspect of the present invention, there is provided an electronic device further including a load circuit having a desired function, in which the load circuit is driven by the power of the power supply means and is also reset by the voltage of the output terminal of the voltage detecting circuit in the case where the voltage of the power of the power supply means is a predetermined voltage or lower. With this structure, there can be adopted the power supply means which supplies a power having a high voltage, and also it is possible to realize the electronic device having the voltage detecting circuit which not only reflects the voltage detection result on the output terminal even if the voltage of the power of the power supply means is lowered, but also is low in the power consumption, and having the load circuit which is not fatally damaged in the systematic or hardware fashion even if the voltage of the power of the power supply means is lowered.